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Full text of "Text Book Of Mechanical Engineering"

Examples on Beams.

Example 25. — Find the safe concentrated load in the following-
cases by the approximate formula.

(1)   Wrought Iron Plate Girder.— Each flange, 10" x J" ; angles?
3 i" x 3i" x i" ; total depth, 3 ft. ;  span, 28 ft. ; /t or/c = 5 tons.
Allow for |" rivets.

(2)   Wrought Iron Rolled Cantilever, H Section.— Each flange,
4i" x f"; depth, 8" ; overhang, 8 ft. ; ft or/c=* 5 tons.

(3)  Cast Iron Girder.— One flange, 3"xr|"; one flange, 9"xiJ";

depth, 12"; span, 20 feet.

"(All from Eng. Exam. 1891 and 1892.)

/ \   iTtr

(2)   W =                       - —   — -__

(3)/c <3C = 3 x ii x 4= 18 tons ; and/t at=9 x i J x i J= 17 tons

„,    4x17/2    4x17x10^-

.-.   W = 3 — '    =1 - i - ^tons      = 2-97 tons.

/              20X12                   - ZŁ- -

Example 26. — Find the depth of an engine guide bar 10" wide
and 4 ft. span. Total piston pressure = 25 tons ; length of connecting
rod = twice stroke ; and greatest obliquity supposed to occur with
guide block at centre of span. /0=5 tons.

(Hons. Mach. Constr. Ex. 1892.)

Draw crank and rod to scale, Fig. 407. Then the forces are as
at A, and the triangle of forces is drawn parallel, as at D.

_.    E     press, on bars        .,                .          1x25.

But — = *— : -   and press, on bars = - - = 6*25 tons
D      piston press.                                       4

4X10X5        -

Example 27. — The girder stays of a combustion chamber are 21
span, and are spaced 8J" centres apart (see Figs. 31 1 and 312), the sec-
tion being rectangular, 5!" deep by if" wide. There are two bolts to
each stay, 7" apart (Fig. 408). Find the greatest stress in the stay when
steam pressure = 225, Ibs, per sq. in. (Hons. Mach. Constr. Ex. 1891.)

The roof plate is equivalent to a continuous girder over three
spans, as at p. 445, so the pull on each bolt would apparently
be ^J- W. But the plate is continuous transversely also. Therefore